Sloped A/D converters have been used in the prior art. A sloped A/D converter may operate in conjunction with a constant current being applied to a capacitor, thereby causing the voltage on the capacitor to continually increase (or decrease). A constant voltage source may be connected across a resistance to set the constant current.
A continually running clock drives a counter that is indicative of the digital value related to the voltage on the capacitor at any time. The counter value is latched at the time that a comparator senses that the ramp value is the same as the sample value. This count represents a digital count of the analog voltage being sampled.
Sloped A/D converters include many advantages including noise independence, simple calibration, and a simple structure.
Other flavors of sloped A/D converters exist, including dual slope A/D converters which integrate both the signal and the reference, in order to increase the accuracy of the system. A ramp-compare ADC (also called integrating, dual-slope or multi-slope ADC) produces a saw-tooth signal that ramps up, then quickly falls to zero. When the ramp starts, a timer starts counting. When the ramp voltage matches the input, a comparator fires, which causes the timer's value to be recorded.